When fabricating integrated circuits, photolithography is often used to form various features such as metal lines into a semiconductor substrate. To form these features, photo-masks are used to form a pattern into a photo-resist layer. The regions where the photo-resist layer is removed expose the underlying substrate to an etching process used to form trenches where metal is subsequently placed.
One type of photolithography is Extreme Ultraviolet (EUV) lithography. In one example of an EUV mask, a patterned absorption layer is formed on a reflective multilayer. To expose a photoresist layer on a substrate, EUV light is projected onto the mask through a number of mirrors. The exposed portions of reflective layer then reflect light onto the substrate on which an integrated circuit is to be formed. The light thus exposes a photoresist layer deposited on that substrate.
An EUV mask typically includes a capping layer between the reflective layer and the absorption layer. The capping layer protects the reflective layer from various particles that accumulate on the mask during field operations. The capping layer, however, is subject to damage as well. For example, the EUV mask is generally cleaned after a certain number of uses. This cleaning process can cause damage to the capping layer over time. Additionally, the capping layer can be subject to oxidation, which also damages the capping layer.